Micro-electromechanical actuator

ABSTRACT

A micro-electromechanical actuator includes a pair of elongate layers capable of being heated with an electrical current to thermally expand and to perform work. A pair of spacers separates the elongate layers from each other. The spacers are arranged at respective opposite ends of the elongate layers and are fast with the layers so that the actuator is deflected when one of the layers is heated.

CROSS REFERENCES

This application is a continuation application of U.S. patent application Ser. No. 09/693,079 filed Oct. 20, 2000, the entire contents of which are herein incorporated by reference.

CO-PENDING APPLICATIONS

Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications/granted patents filed by the applicant or assignee of the present invention with the present application:

6428133 6526658 6315399 6338548 6540319 6328431 6328425 6991320 6854825 6383833 6464332 6439693 6390591 7018016 6328417 6322194 6382779 6629745 09/575197 7079712 09/575123 6825945 09/575165 6813039 6987506 7038797 6980318 6816274 7102772 09/575186 6681045 6728000 7173722 7088459 09/609303 6922779 6978019 09/575181 7068382 7062651 6789194 6789191 6644642 6502614 6622999 6669385 6549935 6727996 6591884 6439706 6760119 09/575198 6290349 6428155 6785016 6831682 6870966 6822639 6737591 7055739 09/575129 6830196 6832717 6957768 09/575162 09/575172 7170499 7106888 7123239 6409323 6281912 6604810 6318920 6488422 6795215 7154638 6924907 6712452 6416160 6238043 6958826 6812972 6553459 6967741 6956669 6903766 6804026 09/575120 6975429

The disclosures of these co-pending applications are incorporated herein by cross-reference.

FIELD OF THE INVENTION

The present invention relates to materials potentially suitable for use as the expansive element in thermoelastic design and to methods for ranking the potential relative suitabilities of those materials.

The invention as developed originally as a means of identifying and ranking a range of materials that potentially may exhibit superior properties for use in the manufacture of microscopic thermal bend actuators for use in micro-electro mechanical systems (MEMS), and will be described hereinafter with reference to this field. However, it will be appreciated that the invention is not limited to this particular use and is equally applicable to macroscopic design even though the overall design considerations are vastly different and certainly less complex.

BACKGROUND OF THE INVENTION

It is important to clarify that thermoelastic actuation is characterized using force, deflection and temperature as opposed to switching, which is characterized using deflection and temperature rise alone. Macroscopic thermoelastic actuators are typically used as switches that activate other more energy efficient actuation systems, however, microscopic thermoelastic actuators are an attractive actuation mechanism for a number of reasons. This includes the down scaling of certain physical phenomena. For example, it is possible to fabricate very thin films that decrease the thermal mass and minimize efficiency losses. Opposing gravitational and inertial forces become negligible on the microscopic scale. Other advantages include ease of fabrication (although more complex than simple electrostatic actuators) and the possibility of low voltage operation. Disadvantages include a low operational bandwidth determined by the thermal conductivities of substrate materials—this is more of an advantage for the current application allowing for rapid firing.

A relatively diverse range of output force and deflection values can be obtained by altering actuator geometry. However, the fundamental operation of actuation is directly related to the mechanical and thermal properties of the component materials. Correct material selection in association with effective design can result in either a smaller or a more efficient actuator. Such an actuator increases wafer yield and is thus more commercially viable. A more efficient actuator may be battery powered increasing operation simplicity and negating the requirement for expensive voltage transformers. An increase in thermal efficiency improves the operational firing frequency, and decreases the possibility of thermal crosstalk. This is especially relevant for arrays of thermal actuators in a micro-cilia device.

However, material selection for MEMS application is not straightforward. Firstly, published thin film properties can vary greatly due to different fabrication methods and difficulties associated with experimentally quantifying material properties on the microscopic scale. Secondly, certain thin films can only be fabricated with certain layer thicknesses because inherent stress can shatter or curl the substrate wafer. Thirdly, only certain materials can be used in the fabrication process at most fabs as the introduction of a new material can contaminate machinery.

Progress to Date

Until recently, the only materials regularly used or considered for use in such applications were polysilicon, single crystal silicon. However, the applicant just previously made the surprising discovery that titanium nitride and titanium boride/diboride exhibited excellent properties relevant to this application.

Realising the breakthrough this surprising discovery signified, the applicant sought to try and identify possible alternatives in order to provide designers of thermoelastic systems with more choice and flexibility. However, given the lack of available data on their film properties for various materials and the fact that empirical testing with MEMS would be prohibitively expensive, there was clearly a need, or it was at least highly desirable to be able to determine a method of evaluating materials for this use based solely on the commonly available macro material properties.

SUMMARY OF THE INVENTION

It is therefore an ultimate object of one aspect of this invention to identify a range of alternative materials that will potentially exhibit superior properties for use in thermoelastic design and of another aspect to provide a means of ranking the potential suitability of a given range of materials for this same use.

According to a first aspect of the invention there is provided a method of selecting a material for use as the expansive element in a thermoelastic design by deriving an indicator of the material's potential effectiveness for that use, said method including the step of calculating a dimensionless constant εγ for that material in accordance with the formula:

${ɛ\;\gamma} = \frac{E\;\gamma^{2}T}{\rho\; C}$ wherein E is the Young's modulus of the material; γ is the coefficient of thermal expansion; T is the maximum operating temperature, ρ is the density and C is the specific heat capacity.

Preferably, the method of selection includes the step of normalising the dimensionless constant relative to that of silicon to a value ε which is achieved by deriving the value εγ for the material of interest at the relevant temperature value and dividing this by the value of ε obtained for silicon at that same temperature.

The relevant maximum operating temperature will depend upon the surrounding materials and their function but is most commonly the oxidising temperature or the melting point temperature.

Desirably, the selection method includes the step of eliminating certain materials by requiring a pre-determined resistivity range. In one preferred form this resistivity range is between 0.1 μΩm and 10.0 μΩm.

In accordance with a third aspect of the invention there is provided an expansive element in a thermoelastic design that is made from any functionally suitable material or combinations of materials selected from a group including:

silicides and carbides of titanium.

In accordance with a fourth aspect of the invention there is provided an expansive element in a thermoelastic design that is made from any functionally suitable material or combinations of materials selected from a group including:

borides, silicides, carbides and nitrides of tantalum, molybdenum, niobium, chromium, tungsten, vanadium, and zirconium.

In accordance with a fifth aspect of the invention there is provided an expansive element in a thermoelastic design that is made from any functionally suitable alloy material or combinations of alloy materials selected from the group including:

borides, suicides, carbides and nitrides of titanium, tantalum, molybdenum, niobium, chromium, tungsten, vanadium, and zirconium.

Preferably the expansive element in a thermoelastic design in accordance with the third, fourth or fifth aspect of the invention also includes one or more of the following properties:

(a) a resistivity between 0.1 μΩm and 10.0 μΩm;

(b) chemically inert in air;

(c) chemically inert in the chosen ink; and

(d) depositable by CVD, sputtering or other thin film deposition technique.

BRIEF DESCRIPTION OF THE DRAWINGS

Derivation of the dimensionless constant ε of the first aspect of the invention, together with sample applications and examples of derived values of this constant and other properties for a range of materials, will now be described in detail with reference to the accompanying drawings in which:

FIG. 1 shows a schematic representation of a thermoelastic actuator;

FIG. 2 shows a plot of longitudinal work versus heat energy for single material clamped/free titanium beam (length 20 μm, thickness 1 μm, width 5 μm);

FIG. 3 shows a plot derived from FIG. 2 of expansion efficiency versus temperature efficiency for a clamped/free titanium beam; and

FIG. 4 shows a comparison of mechanical work versus the heat energy of thermoelastic actuator fabricated from Titanium and Silicon.

DETAILED DESCRIPTION

A non-dimensionalized material actuation efficiency is presented that assesses the potential application of a material to thermoelastic design. The method is based on the material thermal and mechanical properties and assists in a structured approach of material selection for effective design.

The Material Actuation Efficiency

Actuators are characterized by a combination of deflection, force and operation temperature in contrast to switches that are characterized by operation temperature and deflection alone. Fundamental thermoelastic design is characterized by the differential longitudinal expansion of two bonded layers. Thus, the expansion of isolated unbonded layers directly relates to global behaviour. A single material beam is used here to illustrate the material actuation efficiency. The approach is straightforward and relates to general thermoelastic design. The derivation assumes that material properties are constant across the thermal range.

Equations 1 to 3 are fundamental thermomechanical equations describing the behaviour of simple single material beam subjected to a quantity of heat, Q as illustrated in FIG. 1. Equation 1 describes the extension, δL, of a free/free beam and equation 2 describes the reaction force, F, of a clamped/clamped beam. δL=γL₀T  (EQ 1) Where: δL=extension of beam, L₀=original length of beam, T=operation temperature (temperature rise), and γ=coefficient of thermal expansion of beam. F=AEγT  (EQ2) F=force exerted by beam expansion, A=cross sectional area of beam, E=Young's Modulus. Q=VρCt  (EQ3) Where: Q=heat energy input, V=volume of beam, ρ=density, and C=specific heat capacity of beam.

Potential mechanical work is given by equation 4 and is defined as the product of the clamped beam force, F, and free beam deflection, δL. The quadratic relationship between the heat input and output mechanical work for the simple monolithic beam is shown in FIG. 18. W=FδL  (EQ4) Where: W=mechanical work

Equation 5 describes the non-dimensional thermoelastic actuation efficiency and is formulated as the quotient of the mechanical work and heat energy as described by equations 3 and 4. The efficiency is independent of geometry and is a primary indication of a material's potential application to thermoelastic design. The linear relationship between the actuation efficiency and material temperature for the simple beam is shown in FIG. 3. The graph indicates that high temperature operation is desirable for maximum efficiency. The plot is limited by the applicable operation temperature and therefore, different material plots are of different lengths. The assumption used in this text is that the operation temperature is the material melting point because it is indicative of the operable thermal range. Thus, the material actuation efficiency, ε, is defined as the actuation efficiency at the maximum operable temperature, T, of that material. The slope of the efficiency curve is a constant, m_(ε) and is defined in equation 6. The combination of ε and m_(ε) fully characterize a materials actuation characteristics non graphically.

$\begin{matrix} \begin{matrix} {ɛ = \frac{{{Output}\mspace{14mu}{Mechanical}\mspace{14mu}{Work}}\;}{{Heat}\mspace{14mu}{Energy}\mspace{14mu}{Input}}} \\ {= {\frac{E\;\gamma^{2}T}{\rho\; C}\left\lbrack \frac{\left( {N/m^{2}} \right)\left( {1/{{{^\circ}C}.^{2}}} \right)\left( {{{^\circ}C}.} \right)}{\left( {{kg}/m^{3}} \right)\left( {{{Nm}/{kg}}\;{{{^\circ}C}.}} \right)} \right\rbrack}} \end{matrix} & \left( {{EQ}\mspace{14mu} 5} \right) \\ \begin{matrix} {m_{ɛ} = \frac{\mathbb{d}ɛ}{\mathbb{d}T}} \\ {= {\frac{E\;\gamma^{2}}{\rho\; C}\left\lbrack \frac{{N/m^{2}}{1/{{{^\circ}C}.^{2}}}}{{{kg}/m^{3}}{{Nm}/{kg}}\;{{{^\circ}C}.}} \right\rbrack}} \end{matrix} & \left( {{EQ}\mspace{14mu} 6} \right) \end{matrix}$ Material Selection

Different thin film materials including materials with extreme properties (PTFE—high g, Diamond—high E) and compounds from all the major CVD groups including borides, silicides, nitrides and carbides is shown in Table 2. The efficiency values are scaled according to silicon efficiency values because the inclusion of scaled values greatly simplifies design equations described in the following text. The scaling or comparison of a material with respect to a reference material is an integral step in the material selection process. In addition, scaling also results in a more readable index as illustrated by the following comparisons. Silicon is chosen as the reference material because of its predominance in lithographic fabrication.

Preliminary candidates for thermoelastic actuation can be selected according to efficiencies and slopes, however, it is important to note that two materials that have identical ε but differing m_(ε) will output different amounts of work for any constant geometry (see Comparison 1 below, different amounts of heat energy are also required). Three important design parameters are defined here as heat input, work output and volume. A design matrix can be constructed by varying each parameter and can then be used to select suitable materials. The following comparisons are used to assemble the design matrix.

TABLE 2 Material Properties. g E r C m_(e)/m_(r, e) O.T M.P. MN MN KXX R Material 10⁻⁶/° C. GPa kg/m³ J/kg° C. ° C.⁻¹ ° C. ° C. O.T. M.P. W/m · K mWm Aluminum 23.1 68.9 2700 897 17.12 657 7.98 231 0.027 Boron Carbide 4.5 454 2520 955 4.31 2450 7.49 35 5e4 Chromium diBoride 11.1 540 5600 690 19.42 1000 2150 13.78 29.62 32 0.18 Chromium diSilicide 5.9 5600 1150 1560 0.8 Chromium Carbide 9.9 385 6680 530 12.02 1100 1895 9.38 16.16 19 0.75 Chromium Oxide 9.0 102 5210 730 2.45 1000 2603 1.74 4.52 30 13 Copper 16.5 110 8940 386 9.79 1085 7.53 398 0.017 Gold 14.2 80 19300 129 7.31 1064 5.52 315 0.023 Hafnium Carbide 6.3 410 12670 190 7.63 600 3930 3.24 21.25 13 0.4-0.6 Hafnium diBoride 7.6 11200 300 1500 3250 51 0.1 Hafnium diSilicide 8030 1100 1700 Hafnium Monocarbide 6.5 424 11940 3890 8 0.5 Hafnium Nitride 6.5 13,940 500 3300 17 32 Molybdenum 4.8 343 10200 251 3.48 2623 6.48 138 Molybdenum Boride 5 685 7480 530 4.87 1000 2140 3.46 7.40 27 0.18 Molybdenum Carbide 6.7 530 9120 315 9.34 500 2500 3.31 16.56 22 0.57 Molybdenum diSilicide 8.4 450 6240 550 10.44 1700 2050 12.58 15.17 49 0.7 Nickel 13.4 200 8900 444 10.25 1455 10.58 90.7 Niobium diBoride 8.6 650 7210 420 17.91 850 3000 10.80 38.10 0.12 17 Niobium diSilicide 8.5 5690 900 2050 0.5 Niobium Carbide 7.4 450 7820 290 12.26 650 3500 5.65 30.42 14 0.19 PTFE 220 1.3 2130 1024 32.54 200 4.62 140 10e22 Silicon 3.0 162 2330 705 1.00 1410 1410 1 1 149 2300 Silicon Carbide 4.7 304 3440 669 3.29 2700 6.30 90 0.5 Tantalum Carbide 6.7 510 14500 190 9.37 650 3900 4.32 25.93 23 0.35 Tantalum diBoride 8.5 250 12600 250 6.47 850 3090 3.90 14.17 16 0.14 Tantalum diSilicide 9.5 9080 360 800 2670 0.46 Titanium Carbide 7.4 462 4920 480 12.08 700 3160 6.00 27.08 17.2 1.55 Titanium diBoride 8.2 575 4450 632 15.51 1400 3253 15.40 35.78 26.4 0.13 Titanium diSilicide 10.7 270 4100 480 17.72 1300 1540 16.34 19.35 46 0.145 Titanium Nitride 9.4 600 5450 636 17.25 500 2950 6.12 36.10 30 1.35 Tungsten Boride 5.0 790 13100 460 3.70 1000 2365 2.62 6.20 52 0.19 Tungsten Carbide 5.2 690 15800 200 6.66 500 2780 2.36 13.13 29 0.2 Tungsten diSilicide 7.0 300 9750 330 5.15 1200 2165 4.39 7.91 48 33e10 Vanadium diBoride 7.6 260 5100 670 4.96 600 2430 2.11 8.54 42 0.13 Vanadium Carbide 6.7 420 5480 530 7.32 600 2730 3.12 14.18 10 0.59 Vanadium diSilicide 11.2 5100 1000 1700 25 0.66 Vanadium Nitride 8.1 460 6080 630 8.89 450 2170 2.84 13.68 5.2 0.85 Zirconium Carbide 6.3 410 6560 250 11.19 600 3440 4.76 27.31 22 0.42 Zirconium diBoride 5.9 340 6170 1300 3245 58 0.15 Zirconium diSilicide 8.7 270 4900 1150 1600 15 0.76 Zirconium Nitride 5.9 500 7350 400 6.68 500 2950 2.37 13.97 10 0.2-0.3 Where: γ = Coefficient of thermal expansion. E = Young's Modulus, ρ = density, C = specific heat capacity, O.T. = Oxidizing temperature, M.T. = Melting Temperature, m_(ε)= Efficiency Slope (normalized to Silicon m_(ε) value, normalized Silicon value m_((r, ε)) = 0.8865e−06), ε_(c) = Material Index (normalized to Silicon ε value, normalized Silicon ε_(r) = 1.25e−03), KXX = thermal conductivity, and R = resistivity. Comparison 1

The mechanical work and heat input between a material and silicon for a constant beam volume is compared. Thus, Comparison 1 calculates the maximum possible relative work and associated relative heat input required due to a direct material substitution. Details of the comparison for different materials are included in Table 3 which shows that CVD ceramics are far superior actuator materials than silicon (Table 3 is formulated using melting point and Table 4 is formulated using oxidation temperature). Titanium nitride can output 159.3 times more the amount of mechanical work than silicon with only 4.41 times the amount of heat input. The factor in equation 8 and the scaled material efficiency ratio (as included in Table 2) repeatedly occur in the following comparisons illustrating the versatility of the method.

$\begin{matrix} {\frac{W_{c}}{W_{r}} = {\frac{ɛ_{c}Q_{c}}{ɛ_{r}Q_{r}} = {\frac{ɛ_{c}}{ɛ_{r}}\left( \frac{\rho_{c}C_{c}T_{c}}{\rho_{r}C_{r}T_{r}} \right)}}} & \left( {{EQ}\mspace{14mu} 7} \right) \end{matrix}$

The r subscript denotes the reference material which is silicon in this case. The c subscript denotes the compared material.

$\begin{matrix} {\frac{Q_{c}}{Q_{r}} = \left( \frac{\rho_{c}C_{c}T_{c}}{\rho_{r}C_{r}T_{r}} \right)} & \left( {{EQ}\mspace{14mu} 8} \right) \end{matrix}$ Comparison 2

Different materials increase in temperature by different amounts when subjected to the same quantity of heat energy for a constant volume. The material volume is scaled relative to the silicon volume according to the constraints that the same amount of silicon heat energy is input to both actuators and the compared material attains its operational temperature. Thus, the actuation efficiency value remains unchanged because it is not a function of volume and the operable temperature is reached (as equation 5 shows). Comparison 2 represents the design case where heat and volume are critical factors.

The scaled volume and output mechanical work are calculated using equations 9 and 10. The volume change is typically implemented by modifying one geometric dimension, i.e. length, width or thickness. Titanium nitride is capable of 36.1 times the amount of work that silicon is capable with the same heat energy input but with only 0.23 times the volume. Equation 9 is the inverse of equation 8 and equation 10 is simply the scaled efficiency number as included in Table 2.

$\begin{matrix} {Q_{r} = {{V_{r}\rho_{r}C_{r}T_{r}} = {Q_{c} = {\left. {V_{c}\rho_{c}C_{c}T_{c}}\Rightarrow\frac{V_{({c,{Qr}})}}{V_{r}} \right. = \frac{\rho_{r}C_{r}T_{r}}{\rho_{c}C_{c}T_{c}}}}}} & \left( {{EQ}\mspace{14mu} 9} \right) \end{matrix}$

The first entry of the bracketed subscript in these equations refers to the material that the beam is constructed from. The second entry refers to the constraining variable for the described parameter. For example—W_((c,Vc))=Mechanical work output from beam constructed of compared material with a volume of V_(c).

$\begin{matrix} {\frac{W_{({c,{Vc}})}}{W_{({r,{Vr}})}} = {\frac{ɛ_{c}Q_{r}}{ɛ_{r}Q_{r}} = \frac{ɛ_{c}}{ɛ_{r}}}} & \left( {{EQ}\mspace{14mu} 10} \right) \end{matrix}$ Comparison 3

The output mechanical work resulting from silicon heat energy for constant volume beams is compared. The operation temperature and efficiency value for the compared material changes. However, the new efficiency is easily calculated using a multiplicative ratio of the new and old operation temperatures because of the linear relationship between temperature and efficiency (as shown in FIG. 3). The new operation temperature and work are given by equations 11 and 12. This comparison represents the design case where heat is a critical parameter.

PTFE will melt when subjected to the input silicon heat value. Titanium disilicide outperforms titanium nitride mainly because of the higher computed operating temperature (Table 3).

$\begin{matrix} \begin{matrix} {Q_{r} = {{V_{r}\rho_{r}C_{r}T_{r}} = Q_{c}}} \\ {= \left. {V_{c}\rho_{c}C_{c}T_{({c,{Qr}})}}\Rightarrow T_{({c,{Qr}})} \right.} \\ {= {T_{r}\left( \frac{\rho_{r}C_{r}}{\rho_{c}C_{c}} \right)}} \end{matrix} & \left( {{EQ}\mspace{14mu} 11} \right) \end{matrix}$ Comparison 4

$\begin{matrix} \begin{matrix} {\frac{W_{({c,{Qr}})}}{W_{({r,{Qr}})}} = \frac{ɛ_{({c,{Qr}})}Q_{r}}{ɛ_{r}Q_{r}}} \\ {= \frac{T_{({c,{Qr}})}ɛ_{2}}{T_{c}ɛ_{r}}} \\ {= {\left( \frac{\rho_{r}C_{r}T_{r}}{\rho_{c}C_{c}T_{c}} \right)\frac{ɛ_{c}}{ɛ_{r}}}} \end{matrix} & \left( {{EQ}\mspace{14mu} 12} \right) \end{matrix}$ The material volume is scaled with respect to the silicon volume according to the constraint that the compared material operation temperature and silicon work are maintained. Thus, if the silicon work value is less then the original work then the volume is scaled down. Otherwise the volume is increased as is the case for PTFE or amorphous Silicon Dioxide. The material actuation efficiency reoccurs in the calculations as an inverse as shown in equation 14

Titanium nitride can output the same amount of work as silicon but with a volume that is less than two orders of magnitude smaller with an input heat energy that is less than an order smaller.

$\begin{matrix} \begin{matrix} {W_{r} = {{V_{r}E_{r}\gamma_{r}^{2}T_{r}^{2}} = W_{c}}} \\ {= \left. {V_{c}E_{c}\gamma_{c}^{2}T_{c}^{2}}\Rightarrow\frac{V_{({c,{Wr}})}}{V_{r}} \right.} \\ {= \frac{E_{r}\gamma_{r}^{2}T_{r}^{2}}{E_{c}\gamma_{c}^{2}T_{c}^{2}}} \end{matrix} & \left( {{EQ}\mspace{14mu} 13} \right) \\ {\frac{Q_{({c,{Vc}})}}{Q_{({r,{Vr}})}} = {\frac{ɛ_{r}W_{r}}{ɛ_{c}W_{r}} = \frac{ɛ_{r}}{ɛ_{c}}}} & \left( {{EQ}\mspace{14mu} 14} \right) \end{matrix}$ Comparison 5

The input heat energy required to output silicon mechanical work for constant volume beams is compared. The operation temperature and thus efficiency value for the compared material changes. The new efficiency can be calculated in an identical fashion to that described in comparison 3. The operational temperature and heat input values are calculated using equations 15 and 16.

The table shows that titanium disilicide slightly outperforms titanium nitride whereas both PTFE and silicon dioxide will melt. The CVD ceramics are again shown to have the best performance.

$\begin{matrix} {W_{r} = {{V_{r}E_{r}\gamma_{r}^{2}T_{r}^{2}} = {W_{c} = {\left. {V_{c}E_{c}\gamma_{c}^{2}T_{c}^{2}}\Rightarrow T_{({c,{Wr}})} \right. = {\left( \frac{\gamma_{r}}{\gamma_{c}} \right)\sqrt{\frac{E_{r}}{E_{c}}}}}}}} & \left( {{EQ}\mspace{14mu} 15} \right) \\ {\frac{Q_{({c,{Wr}})}}{Q_{({r,{Wr}})}} = {\frac{ɛ_{r}W_{r}}{ɛ_{({c,{Qr}})}W_{r}} = {\frac{ɛ_{r}T_{c}}{ɛ_{c}T_{({c,{Qr}})}} = {\frac{ɛ_{r}T_{c}\gamma_{c}}{ɛ_{c}T_{r}\gamma_{r}}\sqrt{\frac{E_{c}}{E_{r}}}}}}} & \left( {{EQ}\mspace{14mu} 16} \right) \end{matrix}$

TABLE 3 Design comparisons for materials included in Table 2. Comparisons are done using melting point temperature Comparison 2 Comparison 3 Comparison 4 Comparison 5 Comparison 1 Q V, Q W V, W V V_((c, Qr))/ W_((c, Vc))/ W_((c, Qr))/ V_((c, Wr))/ Q_((c, Vc))/ Q_((c, Wr))/ Constant Q_(c)/Q_(r) W_(c)/W_(r) V_((r, Qr)) W_((r, Vr)) T_((c, Qr)) W_((r, Qr)) V_((r, Vr)) Q_((r, Vr)) T_((c, Wr)) Q_((r, Wr)) Aluminum 0.69 5.48 1.46 7.98 >Tmelt 0.183 0.125 280.79 0.29 Boron Carbide 2.55 19.06 0.39 7.49 962.41 2.94 0.053 0.133 561.51 0.58 Chromium diBoride 3.59 106.23 0.28 29.62 599.41 8.26 0.009 0.0330 208.73 0.35 Chromium Carbide 2.90 46.80 0.35 16.16 654.20 5.58 0.021 0.062 277.16 0.42 Chromium Oxide 4.27 19.34 0.23 4.52 608.98 1.06 0.052 0.221 592.32 0.97 Copper 1.62 12.18 0.62 7.53 671.18 4.66 0.082 0.132 311.11 0.46 Gold 1.14 6.31 0.87 5.52 930.29 4.82 0.159 0.181 423.90 0.46 Hafnium Carbide 4.08 86.81 0.24 21.25 962.13 5.20 0.012 0.047 422.05 0.44 Molybdenum 2.90 18.78 0.34 6.48 904.67 2.23 0.053 0.154 605.63 0.67 Molybdenum Boride 3.66 27.09 0.27 7.40 584.23 2.02 0.037 0.135 411.42 0.70 Molybdenum Carbide 3.10 51.36 0.32 16.56 806.23 5.34 0.019 0.061 349.05 0.43 Molybdenum diSilicide 3.04 46.09 0.33 15.17 674.86 4.99 0.022 0.066 302.14 0.45 Nickel 2.48 26.26 0.40 10.58 586.13 4.26 0.038 0.095 284.10 0.48 Niobium diBoride 3.92 149.44 0.25 38.10 764.86 9.71 0.007 0.026 245.55 0.32 Niobium Carbide 3.43 104.26 0.29 30.42 1021.31 8.88 0.010 0.032 342.97 0.34 PTFE 0.19 0.87 5.31 4.62 >Tmelt 1.152 0.216 >Tmelt Silicon 1.00 1.00 1.00 1 1410.00 1.00 1.000 1 1410.00 1.00 Silicon Carbide 2.68 16.91 0.37 6.30 1006.42 2.35 0.059 0.158 657.00 0.65 Tantalum Carbide 4.64 120.27 0.22 25.93 840.70 5.59 0.008 0.038 355.83 0.42 Tantalum diBoride 4.20 59.57 0.24 14.17 735.28 3.37 0.017 0.071 400.60 0.54 Titanium 1.70 7.27 0.59 4.28 984.12 2.52 0.138 0.234 619.87 0.63 Titanium diBoride 3.95 141.32 0.25 35.78 823.54 9.06 0.007 0.028 273.81 0.33 Titanium diSilicide 1.31 25..32 0.76 19.35 1176.90 14.79 0.040 0.0517 306.22 0.26 Titanium Nitride 4.41 159.36 0.23 36.10 668.21 8.18 0.006 0.0277 233.83 0.35 Tungsten Boride 6.15 38.16 0.16 6.20 384.36 1.01 0.026 0.161 383.10 1.00 Tungsten Carbide 3.79 49.80 0.26 13.13 732.95 3.46 0.020 0.076 394.10 0.54 Tungsten diSilicide 3.01 23.80 0.33 7.91 719.86 2.63 0.042 0.126 444.06 0.62 Vanadium diBoride 3.58 30.63 0.28 8.54 677.83 2.38 0.033 0.117 439.34 0.65 Vanadium Carbide 3.42 48.53 0.29 14.18 797.46 4.14 0.021 0.071 392.10 0.49 Vanadium Nitride 3.59 49.09 0.28 13.68 604.67 3.81 0.020 0.0731 309.91 0.51 Zirconium Carbide 2.44 66.51 0.41 27.31 1412.28 11.21 0.015 0.0366 422.05 0.30 Zirconium Nitride 3.74 52.32 0.27 13.97 787.80 3.73 0.019 0.0716 408.09 0.52

TABLE 4 Design comparisons for material included in Table 2. Comparisons are done using oxidation temperature Comparison 2 Comparison 3 Comparison 4 Comparison 5 Comparison 1 Q V, Q W V, W V V_((c, Qr))/ W_((c, Vc))/ W_((c, Qr))/ V_((c, Wr))/ Q_((c, Vc))/ Q_((c, Wr))/ Constant Q_(c)/Q_(r) W_(c)/W_(r) V_((r, Qr)) W_((r, Vr)) T_((c, Qr)) W_((r, Qr)) V_((r, Vr)) Q_((r, Vr)) T_((c, Wr)) Q_((r, Wr)) Vanadium diBoride 0.885 1.864 1.13 2.10 >T oxid. 0.326 0.475 439.337 0.648 Vanadium Carbide 0.752 2.341 1.33 3.11 >T oxid. 0.26 0.32 392.1 0.49 Vanadium Nitride 0.74 2.1 1.34 2.83 >T oxid. 0.289 0.353 309.9 0.513 Zirconium Carbide 0.425 2.02 2.35 4.75 >T oxid. 0.301 0.21 422.05 0.299 Zirconium Nitride 0.64 1.5 1.57 2.36 >T oxid. 0.405 0.423 408.1 0.518 A Thermoelastic Actuator

A hot arm/cold arm actuator is presented in FIG. 1 to illustrate the results contained in Table 3. Only the steady state solution for a quantity of heat input to the heater is analyzed. The device comprises two identical material layers separated by air and connected to each other at the ends by a thermally non-conductive block. The force/deflection characteristics of the output mechanical power can be tuned by altering the separation between the two layers. A greater separation increases the transverse force but decreases deflection.

Two actuators constructed from titanium and silicon are compared using graphed energy results in FIG. 4. Five design comparisons for Titanium are plotted according to the results contained in Table 3. The relationship between volumes, mechanical work and heat energy are identical to those included in Table 3. Titanium volumes are scaled using length for Comparisons 2 and 4.

Discussion

The combination of five separate material properties is important in assessing a material's potential for thermoelastic design and materials with one predominant property have been shown to not necessarily be the best candidate. This is evident in both Table 3 for PTFE (high g) and diamond (high E). Both gold and copper have high g values but are hindered as good candidates by low E and high r values. Silicon is very inefficient compared to certain other materials, however, amorphous silicon dioxide is possibly the most inefficient material of all.

Output mechanical work, input heat energy and actuator volume are three essential characterizing parameters for thermoelastic design. The design method described incorporates these parameters using only material properties and provides a structured approach for material selection. The method is versatile because the approach assesses the potential of a material using easily calculated comparison ratios. It is important to note that the approach is a measure of a materials potential and must be used as a tool in conjunction with other appropriate design criteria. For example, criteria such as force/deflection characteristics of the output work, material resistivity, environmental ruggedness and material availability may be important. The operable temperature range is assumed to be from 0 degrees to the melting point on the Centigrade scale because it is indicative of the material thermal range. However, the maximum operable temperature could be different due to oxidation of the material or other thermal design constraints. Titanium nitride has close to the highest actuation efficiency value when melting point is used as a criteria. However, Titanium diSilicide is potentially a better candidate for use when oxidation temperature is used. Titanium nitride is a practical candidate because it is well established as a CMOS barrier material. The oxidation temperature of TiN can be raised from 500° C. to 900° C. by alloying with aluminum. The alloyed material has a symbol (Ti,Al)N.

The actuation efficiency of a simple thermoelastic titanium beam is low compared to other actuation mechanisms (less than 1 percent). It is theoretically possible to get a thermoelastic actuation efficiency of about 4.5 percent for a simple titanium nitride beam, however, this value typically decreases when the material is implemented in a MEMS device due to associated operational losses (for example—thermal conduction into the substrate).

The invention has been described herein by way of example only. Skilled workers in this field will readily recognise many variations and modifications which do depart from the spirit and scope of the broad inventive concept. 

1. A micro-electromechanical actuator that comprises: a pair of elongate layers capable of being heated with an electrical current to expand thermally and to perform work; and a pair of spacers which separate the elongate layers from each other and are arranged at opposite ends of the elongate layers, the spacers being fast with the layers so that the actuator is deflected when one of the layers is heated; wherein one of the layers which is heated is composed of a material selected by calculating a dimensionless constant ε for the material in accordance with the formula: $ɛ = \frac{E\;\gamma^{2}T}{\rho\; C}$ where E is the Young's modulus of the material; γ is the coefficient of thermal expansion; T is the maximum operating temperature, ρ is the density and C is the specific heat capacity, wherein each spacer is composed of a thermally non-conductive material.
 2. A micro-electromechanical actuator as claimed in claim 1, wherein each elongate layer is composed of a thermally conductive material.
 3. A micro-electromechanical actuator as claimed in claim 1, wherein each spacer is substantially block shaped.
 4. A micro-electromechanical actuator as claimed in claim 1, wherein the layers are composed of a substantially identical material.
 5. A micro-electromechanical actuator as claimed in claim 1, wherein each elongate layer has a resistivity between 0.1 μΩm and 10.0 μΩm and is chemically inert in air. 